Patent Literature 1 has proposed a semiconductor device including a HEMT (High electron mobility transistor), which is a field effect transistor, as a lateral switching device having a heterojunction structure with GaN.
The semiconductor device is provided with a lateral HEMT having a heterojunction structure made of GaN and AlGaN. Specifically, the semiconductor device has, on a substrate, a GaN-based semiconductor layer that includes a GaN electron transit layer and an AlGaN electron supply layer laid on top of another. The AlGaN electron supply layer is formed with a recessed portion, and the thickness of the AlGaN electron supply layer is reduced at the recessed portion. A gate electrode is disposed in the recessed portion. A source electrode and a drain electrode are disposed on the AlGaN electron supply layer on opposite sides of the gate electrode. The recessed portion has a gate embedding portion in which the gate electrode is disposed. The recessed portion further has a first recessed portion and a second recessed portion formed outside of the gate embedding portion, the first recessed portion being adjacent to the source electrode and the second recessed portion being adjacent to the drain electrode.
In the HEMT having such a structure, two-dimensional electron gas (hereinafter referred to as the 2 DEG) carrier is induced, by a piezoelectric effect and a spontaneous polarization effect, below the AlGaN electron supply layer, which are located on the opposite sides of the gate electrode. A current flows between the source and the drain through the 2 DEG carrier and a channel portion provided by a surface layer portion of the GaN electron transit layer under the gate electrode.
In such a HEMT, the thickness of the AlGaN electron supply layer is reduced by forming the recessed portion. Therefore, the stress is more reduced at the thin portion of the AlGaN electron supply layer where the recessed portion is formed than a thick portion of the AlGaN electron supply layer where the recessed portion is not formed. As such, an occurrence of the piezoelectric polarization is suppressed, and a 2 DEG density (hereinafter referred to as Ns) can be reduced. Accordingly, it is less likely that the blocking breakdown voltage will be reduced.
Further, there is known a structure that further has a first AlGaN cap layer and a second AlGaN cap layer on the AlGaN electron supply layer. The first AlGaN cap layer has an Al mixed crystal ratio smaller than that of the AlGaN electron supply layer, and the second AlGaN cap layer has an Al mixed crystal ratio greater than that of the first AlGaN cap layer.
In such a structure, the electron density Ns of the channel, which is the region only made of the single layer of the AlGaN electron supply layer, under the gate electrode, is not reduced, whereas the electron density Ns of the region made of the two layers including the first AlGaN cap layer and the AlGaN electron supply layer is reduced. That is, in the region only made of the single layer of the AlGaN electron supply layer, the 2 DEG carrier is formed based on positive fixed charge existing at a lower position of the AlGaN electron supply layer, resulting in a predetermined Ns. On the other hand, in the region made of the two layers including the first AlGaN cap layer and the AlGaN electron supply layer, negative fixed charge is generated in the surface layer portion of the AlGaN electron supply layer due to the first AlGaN cap layer. In this region, therefore, the positive fixed charge at the lower position of the AlGaN electron supply layer is offset, and thus the Ns is reduced.
The Ns is not reduced in the region made of three layers, which includes the first AlGaN cap layer, the second AlGaN cap layer and the AlGaN electron supply layer. Namely, in the region made of the three layers, since the influence of the fixed charge of the first AlGaN cap layer is reduced due to the positive fixed charge at the lower position of the second AlGaN cap layer, the Ns is not reduced.
As described above, since both the region where the Ns is reduced and the region where the Ns is not reduced are formed, the degradation of the blocking breakdown voltage is suppressed, and an increase in the on resistance can be suppressed.